Accurate sizes: Support relative alignment widths

[joemcgill]

Summary

This is an initial concept for supporting relative alignment widths as described in #1511 (comment).

Relevant technical choices

If the layout setting ends in a px value, compare directly with the image width. If the layout setting uses any other value, use a CSS min() function in sizes.

This also improves the logic for left/right alignment values so they are constrained by wide layouts if the image width is larger than the container.

To test

1. Visit the site editor
2. Edit Styles > Layout
3. Set content width to a relative value, like 30rem
4. Add an image block with default alignment to a post
5. Inspect that the generated sizes value looks similar to sizes="(max-width: min(20rem, 1024px)) 100vw, min(20rem, 1024px)"

[joemcgill]

Thanks for fixing those indents, @mukeshpanchal27! Some times my editor's auto-formatting takes on a life of it's own 😅

I'm going to add some unit tests for this (current tests are still passing) and then I this should be ready for review as well.

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Co-authored-by: joemcgill <joemcgill@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>

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